1. Field of the Invention
The present invention relates to a display control device for transferring video data to a CRT (Cathode Ray Tube) display and a LCD (Liquid Crystal Display) display.
2. Description of the Related Art
In the field of general-purpose computers such as a personal computer (PC), an office computer, etc., there has hitherto been a notebook-type PC displaying pictures or images on a LCD display by use of a display control device (a LCD controller) being built-in a LCD interface and a personal computer/office computer that is digitally connected to the LCD display.
As a recent tendency, there have increased cases of supplying a market with, as main commercial articles, the notebook type PC using the LCD display and the desktop type PC that is adopted the LCD as standard equipment. Further, by reduction of prices of the PCs, a domestic-utility-rate (percentage) of the PCs rises year after year.
The standard of VCCI (Voluntary Control Council for Interference by Information Technology Equipment) applied to the PCs utilized in home, is required a strict standard “CLASS-B”. In “CLASS-B”, a countermeasure against EMI (Electromagnetic Interference) noises due to electromagnetic radiation from the PC is an indispensable item.
FIGS. 7 and 8 are diagrams showing an example of a LCD controller mounted on the PC. The PC using the LCD (which implements a display interface connector for the LCD) as a display device, generally further implements a display interface connector for a CRT display, whereby a picture or an image can be, in the case of both of the LCD and the CRT being connected to the PC, displayed simultaneously on the LCD and the CRT.
Therefore, the LCD controller includes a data converting unit 51 and a FIFO (first-in first-out) buffer 52 as a system (a LCD control unit) related to the display control of the LCD, and a FIFO buffer 53 and a DAC (Digital to Analog Converter) 54 as a system (a CRT control unit) related to the display control of the CRT. The LCD control unit and the CRT control unit are connected to an external video memory (VRAM) for storing data of the picture displayed on the LCD and/or the CRT. Moreover, the LCD controller includes a timing generator 55.
The timing generator 55 receives a basic clock CLK from an external clock generation source (a clock generator), and generates a readout timing signal from the basic clock CLK. The video (picture or image) data are read from the VRAM in accordance with this readout timing signal and inputted to the LCD control unit and the CRT control unit, respectively. In the LCD control unit, the data converting unit 51 converts the video data from the VRAM into a format in which the video data should be transferred to the LCD, and the converted video data are transferred to the LCD via a LCD interface. In the CRT control unit, the DAC 54 effects a digital-to-analogue conversion of the video data from the VRAM, and the converted video data are transferred to the CRT via a CRT interface. These operations are conducted based on a transfer timing signal generated from the basic clock CLK by the clock generator 55.
In such a configuration, the LCD controller has a function of transferring and displaying the same display screen on the CRT and on the LCD as well. Further, the LCD controller has a function of simultaneously displaying the same display screen on the LCR and the CRT. These functions are indispensable functions possessed by the LCD controller as a product.
Incidentally, in designs of the recent PCs adopt a method, as the mainstream method, to reduce the EMI noises from a single mother board, a clock buffer having a spread spectrum function is used as a clock generator of the mother board and supplying spread spectrum clocking (SSC) signals to a CPU and peripheral circuits (a main memory, etc., excluding the LCD controller) thereof which are mounted on the mother board. The SSC signal is a clock signal in which a frequency bandwidth of an original clock signal is spread up to a broad frequency bandwidth that is several ten times as broad as the former frequency band. The SSC signal has a characteristic that a peak of its energy, though the bandwidth thereof expands, decreases, and hence it is hard to emit the EMI noises as compared with the original clock signal.
In the present situation, other clock generator different from the clock generator of the mother board is prepared for the LCD controller, and the LCD controller receives one clock signal (with non-spread spectrum), as the basic clock CLK, in which a cycle of being supplied from the other clock generator is fixed and stable. Accordingly, the LCD controller transfers the basic clock CLK with non-spread spectrum together with the video data to the LCD. This is derived from the following reasons.
(1) If the basic clock signal is the SSC signal, a jitter held on the SSC signal brings about a fluctuation of transfer timing, resulting in an occurrence of a problem of causing a distortion and a flicker on a display screen of the CRT that is analogously operated (which executes a time-division transfer and a display process).
(2) In the case of simultaneously displaying the same picture or image (video) on the LCD and the CRT, it is considered that the video data are read out at different clocks (readout timing) twice from the VRAM and supplied to the LCD control unit and the CRT control unit, respectively. A readout speed from the VRAM in the present situation does not yet reach an extent capable of responding to the readout control under which a frame cycle of the LCD does not conflict with a frame cycle of the CRT. Therefore, in the case of performing the simultaneous display, the frame frequencies of the LCD and of the CRT are made coincident, and the LCD/CRT control units are supplied with the video data read from the VRAM by one reading operation at the readout timing generated from the basic clock CLK.
A tendency over the recent years is that there have increased the number of displayable colors, a resolution and a frame frequency of the LCD, respectively. This is accompanied by a rise in data size of the data transferred to the LCD from the LCD controller and a rise in the frequency of the transfer clock signal transferred to the LCD. Derived from this, a cable for transferring video data signals (R/G/B data signals), synchronous signals (horizontal/vertical synchronous signals) and a transfer clock signal (transfer timing signal) to the LCD from the PC, becomes one of portions where the EMI noises are emitted. In particular, the transfer clock signal becomes a large factor for causing the EMI noises.
The cable is extended between, if the PC is of a notebook type, a box body on the side of the PC main body and a box body of the LCD, these box bodies being connected by hinges (see FIG. 8), and, if the PC is of a desktop type, the PC is in a state of being non-covered with the box body. Thus, the cable is in a position irresistible against electromagnetic radiation, and therefore a PC designer has a large problem of how the radiation of the EMI noises from the cable can be restrained.